Steam generator



y 4, 1963 w. H. ARMACOST 3,089,467

STEAM GENERATOR Filed March 18, 1959 4 Sheets-Sheet 1 g Superheoi WilburH. Armucost ATTORNEY May 14, 1963 Filed March 18, 1959 W. H. ARMACOSTSTEAM GENERATOR Fig. 2.

4 Sheets-Sheet 2 INVENTOR Wilbur H. Armclcost ATTORNEY 1963 w. H.ARMACOST 3,089,467

STEAM GENERATOR Filed March 18, 1959 4 Sheets-Sheet 3 Superheot INVENTORWilbur H. Armucost ATTORNEY May 14, 1963 Filed March 18, 1959 W. H.ARMACOST STEAM GENERATOR 4 Sheets-Sheet 4 Water Well Panel 4 F I Radiants. H. Panel I Flow; down 8| up gi g- L: 3 o: urner e E "s T, I w 2 WaferWall 5 -e 7 Panel e E 3 FM; 0 2: 0 1: 0 u i z *5 5 P 2 Superheaf 5lClnLme ,5 a5 61? w Furnace 3-, Burner c 5 35 g 64 8 O "4- 'g f; g g 5Water Wall %r g g s E W Panel g 5 S 2 To Radlant E G L o 5 S.H.Panel O"i Burner Radiant S. H. Panel 36 Flow down 8| up WaterWall 76 78 80 84 pl 24 Drum 20 22 24\ fig /8 From Turbine 8 I\ L: 7 g Water Wall Panel FRadiant Reheat Panel -I- c =F|w up g[ l '13 J g 28 Burner l. o I u E E 33 5 Water Wall 5 3 2 3 Panel 0- o 4I I: 5 (05' q 0 '0 g 1: o I 2: 21 oI0 Reheat 3 ,Clr.L|ne 2-0- F m 5, (I) E 62 Furnace Burner E 3 2 E 2- :3E Wafer Wall E g 3 5 g 2 Panel 5 5 5 g .9 52 LL 0 2 a a8 Burner RadiantReheat Panel 32 Flow up Water Wall L 76 78 82 84 Panel From TurbineINVENTOR Fig. 4.

Wilbur H. Armacost i A4 KW ATTORNEY 3,889,467 STEAM GENERATOR Wilbur H.Armacost, Scarsdale, N.Y., assignor to Cornhustron Engineering, Inc, NewYork, N.Y., a corporation of Delaware Filed Mar. 18, 1959, Ser. No.896,316 (Iiaims. (El. 122--24) This invention relates generally to vaporor steam generators and has particular relation to high capacity hightemperature and pressure generators as employed in the utility field.

The invention is directed to an improved steam generator organizationemploying so-called twin or separate furnaces which are independentlyfired but have their heat exchange surface interrelated and their steamgenerating surface interconnected with a common steam and water drum soas to form a steam generating organization or unit which may serve asingle turbine generator unit. As embodied, one of the furnaces may betermed a reheat furnace since all of the reheating of the unit isaccomplished in this furnace and the other furnace may be termed asuperheat furnace since all of the radiant superheating surface iscontained in this furnace. The reheat and superheat surfaces or, inother words, the reheat and superheat tubes are arranged in such amanner as to minimize the requirement for interconnecting piping whichmerely conveys the reheat or superheat steam from one section of reheator superheat surface to another or to the turbine.

Each of the furnaces is vertically elongated and positioned inrelatively closely spaced side by side relation, being fired adjacentits lower portion and having combustion gases passing through a suitableopening in the upper end of its rear wall. Extending from this openingis a suitable gas pass arranged in conventional manner so that itextends a short distance horizontally and then vertically downward inparallel relation with the particular furnace. In the upper end of eachof these gas passes is positioned convection superheating surface whichreceives from the steam and water drum steam generated in the steamgenerating portion of the unit. This convection steam heating surface ineach of these gas passes communicates with radiant steam heating surfacedisposed along the various walls of the superheater furnace. Thearrangement is such that the steam from the convection steam heatingsurface enters this radiant steam heating surface at the upper end ofthe boiler, passes in a particular manner through this surface,traversing the full length of the boiler several times and finally iscollected in a suitable outlet header at the lower end of the furnacefrom which it is conveyed to a turbine, which, of course, is alsolocated in proximity to the lower end of the steam generator as isconventional.

The reheat furnace has various of its walls lined with radiant reheattubes and the reheat steam, i.e., the steam to be reheated, is receivedin a suitable header at the lower end of the reheat furnace whichcommunicates with the lower end of various of these tubes with thereheat steam passing up through these tubes and then down through othertubes where it is collected at an outlet header also located at thebottom of the reheat furnace.

Each of these furnaces is fired by horizontally arranged burners thatproject fuel through the front wall of the furnace and there are aplurality of vertically extending horizontally spaced tubular panels ofsteam generating tubes that extend from this front wall a short distanceinto the furnace with these panels dividing this portion of the furnaceinto chambers and with the burners being arranged to horizontallyintroduce fuel and air into these chambers. Each of the furnaces isindependently fired and is provided with a hopper bottom formed byinclining the lower portion of the side walls of the furnace towardPatented May 14, 1963 one another and these side walls are lined withtubes which do not form part of the finishing stages or final portion ofeither the superheater or reheater so that the temperature will not beobjectionally high so as to cause slagging difficulties.

Accordingly, it is an object of this invention to provide an improvedsteam generating unit operating on the reheat cycle having separatedfurnaces and with the steam heating, both reheat and superheat, surfacebeing organized in an eflicient and expeditious manner so thatconnection piping is necessarily reduced to a minimum while at the sametime the general arrangement of the steam heating surface is efficientand conducive to troublefree operation of the unit.

Other and further objects of the invention will become apparent to thoseskilled in the art as the description pro ceeds.

With the aforementioned objects in view, the invention comprises anarrangement, construction and combination of the elements of theinventive organization in such a manner as to attain the results desiredas hereinafter more particularly set forth in the following detaileddescription of an illustrative embodiment, said embodiment being shownby the accompanying drawing wherein:

FIG. 1 is a vertical sectional view looking from the front of the unitand showing the superheat and reheat furnaces in spaced side by siderelation with the burners and the steam generating partitions, whichwould normally not be shown in this section, being indicated in dottedlines;

FIG. 2 is a vertical sectional view of the reheat furnace organizationlooking from the side of the furnace;

PEG. 3 is a view similar to that of FIG. 2 but showing the superheatfurnace; and 7 FIG. 4 is a diagrammatic representation in the nature ofa flow diagram with the various elements being labeled.

Referring now to the drawings, wherein like reference characters areused throughout to designate like elements, the illustrated andpreferred embodiment of the invention depicted therein comprises a vaporor steam generating unit having a pair of separately spaced verticallyelongated furnaces with one of the furnaces being identified as 10 andindicated as the reheat furnace and the other identified as 12 andindicated as the superheat furnace. The steam generating surface of thisunit comprises vertically extending steam generating tubes that line thefront wall 14 of furnace 10 and the front wall 16 of furnace 12 as wellas a portion of the side walls of each of these furnaces and alsoforming part of the steam generating surface are a pair of verticallyextending tubular panels 52 that project into the furnace at the lowerportion thereof and adjacent the front wall.

The steam generating circuit of the unit includes the steam and waterdrum 18 which is common to both of the furnaces and downwardly fromwhich extends the downcomer 20 with this downcomer being connected atits lower end to pump 22. Extending from this pump are supply conduits24 that are connected with headers 26 at the lower end of each of thefurnaces 10 and 12. Extending up from each of these headers are steamgenerating tubes 23 which line the front walls 14 and 16 of the furnaceswith these tubes extending up along these walls and across the roof ofthe furnace and being connected with header 30. The side walls 32 and 34of furnace 10 as well as the side walls 36 and 38 of furnace 12 havetheir forward portion thereof lined with steam generating tubes 39 for adistance identified as 46 in FIGS. 2 and 3. These tubes extend up alongthese side walls and are connected at their upper ends with header 42which is in turn connected by means of conduits 44 to header 30 and withthis header being connected with conduits 4-6 to the steam and waterdrum .18. The front-to-back dimensions of the furnaces and 12 aregreater at the lower portion of the furnace than at the upper portion asclearly indicated in FIGS. 2 and 3, and the tubes 39 that extend alongthe side walls of the upper portion are bifurcated generally at theincline 48 of the front Walls of the furnaces so that the number oftubes extending along the front portion of the side walls below thisincline will be sufiicient to cover this portion of the furnace wall. Inaddition to the steam generating tubes 28 and 39 that line the front andside walls, respectively, of the furnaces 1i) and 12, each of thefurnaces is provided with a pair of panels 52 that project into thefurnace below the inclined front wall portion 48 with these panelsextending generally down from this inclined front wall portion orextending forwardly or inwardly from the front wall as indicated inFIGS. 2 and 3.

1e tubes that make up these panels 52 may advantageously be formed bybifurcating the tubes lining the upper portion of the front furnacewall. The tubes of these panels extend out through the front wall of theparticular furnace adjacent the lower end thereof and then down so thatthey connect with headers 26. The lower ends of tubes 39 on the sidewalls are connected with one of the supply headers 54 which is, in turn,connected by conduit 56 to header 26 so that pump 22 is effective toforce water through all of the steam generating tubes and the steam andwater mixture that egresses from these tubes passes to and is collectedin steam and water drum 18.

Each of the furnaces 1t) and 12 is fired by means of a plurality ofhorizontally spaced rows of burners 58 and, as embodied there are fourverticaly disposed burners in each row and there are three rows arrangedso that the fuel together with combustion supporting air is projectedhorizontally between the panels 52 and between these panels and the sidewalls, or in other words, into the chambers formed by the panels. Thecombustion gases thus generated pass up through the furnace and outthrough outlet 60 located in rear wall 62 of furnace 10 and rear wall 64of furnace 12. Extending from this outlet is a suitable and conventionalgas pass which includes the horizontally disposed portion 66 andvertically disposed portion 68 that extends downwardly and parallel withthe furnace and may communicate with a suitable stack. In the lowerregion of this gas pass portion 63 there is provided the usual heatexchange equipment which may include economizer surface 70 while in theupper portion thereof is positioned the convection superheating surface72. This surface positioned in the gas pass of each furnace is in theform of conventional sinuously disposed tubes with steam being suppliedthereto through conduit 74 and with the inlet header of this steamheating surface being identified as 76. The steam passes up through thissurface to outlet header 78 and from this outlet header the steam passesthrough the additional convection steam heating surface 80 disposed inthe horizontal gas pass portion 66 with the inlet header of this surfacebeing identified as 82 and the outlet header being identified as 84.From outlet header 84 the steam is conveyed to the upper end of thesuperheat furnace 12 and particularly to the inlet header 86 one ofwhich is disposed immediately above each of the side walls 36 and 38 ofthis superheat furnace. These side walls 36 and 38 are lined withsuperheater tubes 88 for the lateral distance identified as 90 (FIG. 3)and every other tube 38 is connected at its upper end with the adjacentinlet header 86 whereby steam flows down through these alternate tubeson the side walls 36 and 38. The lower end of tubes 88 are connectedwith header 92 so that this downwardly flowing steam upon reachingheader 92 flows up through the remaining or alternate tubes 88 whichhave their upper ends connected with the adjacent leg of U-shaped header92 that is disposed above the particular wall. Thus the steam flows downthrough and then up through alternate of the tubes 38. U-shaped header92 is connected with the upper end of tubes 94 that line the rear wall64 of furnace 12 so that the steam flows down through these tubes 94 andthe lower end of 4 these tubes is connected with the final outlet header96 of the superheater from which the steam is conveyed to the turbinewhich, as is conventional, is located in proximity to the lower end ofthe steam generator.

After having a portion of its energy utilized or removed the steam isreturned to the unit for reheating, and as embodied, reheat furnace 10has its side walls lined with tubes 98 and which are connected at theirlower ends with one of the headers 160 there being one such headeradjacent the lower end of each of the side walls 32 or 34. The steam tobe reheated passes from the inlet header 100 up through the tubes 98lining the side walls to the header 102. The upper ends of tubes 98 areconnected at their upper ends with the adjacent leg of U-shaped header102 which has its center portion 106 disposed above rear wall 62 andconnected with the tubes 198 extending downwardly therealong. Thus steamflows up through tubes 98, through header 102 and down through tubes108. This steam is collected in outlet header 110 to which the lowerends of tubes 198 are connected and which is disposed adjacent the lowerend of wall 62.

It will be seen with this organization that there is very littleconnecting piping required solely for the purpose of conveying hightemperature and high pressure steam from one steam heating section toanother and it will be particularly noted that when the steam has beenheated to at least an appreciable amount or extent of its desiredtemperature the requirement for connecting piping which wouldnecessarily operate at a relatively high temperature is at a minimum.This is extremely desirable and important since this high temperature,high pressure piping is very costly.

With regard to the superheat steam, after leaving the intermediate stageof the superheater, i.e., the superheat surface identified as 80, andentering the radiant portion there is substantially no connecting pipingrequired. Thus, with regard to the high temperature end of the superheatrange connecting piping to convey the superheat steam from one sectionof the superheater to another is substantially eliminated andfurthermore the outlet of the superheater is at the bottom of thefurnace and accordingly near the point of use, i.e., the turbine, sothat the extremely long pipe which is often utilized to convey the veryhighest temperature and pressure steam from the upper end of one ofthese units to the turbine is not required.

Likewise, with regard to the reheat furnace interconnecting piping isalso substantially eliminated and the inlet and outlet of the reheaterare located near the source of reheat steam and the point of use of thesteam, i.e., the lower end of the reheat furnace, so that here also,there is no requirement for a long run of pipe to convey the reheatsteam to or from the upper end of the unit.

By referring to the flow diagram of FIG. 4 it is believed that the flowpath of the various fluids and the advantages obtained thereby will bevery evident.

Both reheat furnace 10 and superheat furnace 12 are of the hopper bottomtype where the side walls are sloped inwardly at their lower ends toform a restricted opening 112. The arrangement of the surfaces in thesefurnaces is such that the tubes that line and make up the inclined wallsof the hopper do not have the highest temperature steam passed throughthem, or, in other words, are not the final steam heating stages ofeither the reheat or the superheat furnaces. Accordingly these tubeportions that make up this sloping hopper are relatively cool with thisbeing desirable so that ash particles that slide down these slopes willhave less tendency to adhere to them than would otherwise would be thecase. With regard to the reheat furnace 10, tubes 98 that make up thegreater portion of the hopper, are the tubes that initially receive thesteam to be reheated and accordingly are the coldest portion of theentire reheat surface. Of course the steam generating tubes 39 that formthe front portion of the side walls 32 and 34 are at a relatively lowtemperature. Also with regard to superheat furnace 12 the tubes 88 thatform a substantial portion of the sloping walls of the hopper are thefirst portion of the radiant section of the superheater with the finalportion or finishing section of the superheater being the tubes 94 thatline the rear wall 64 of the superheat furnace. Accordingly the steampassing through the tubes 88 and the tubes themselves are not at thehigh final superheat temperature but are relatively low intermediatetemperature. Here also the tubes 39 that line the front portion of theside walls 36 and 38 of the superheat furnace are a relatively lowtemperature.

It will thus be seen that with the particular organization of thisinvention a novel circuit arrangement and interconnection of heatexchange surface is provided so as to produce an efficiently operatingunit.

While I have illustrated and described a preferred embodiment of myinvention it is to be understood that such is merely illustrative andnot restrictive and that variations and modifications may be madewithout departing from the spirit and scope of the invention. Itherefore do not Wish to be limited to the precise details set forth butdesire to avail myself of such changes as fall within the purview of myinvention.

What is claimed is:

l. A vapor generator operating on the reheat cycle comprising a pair ofupright furnaces generally polygonal in transverse section and disposedin side by side spaced relation, one of said furnaces comprising areheat furnace and the other a superheat furnace, said furnaces havingcombustion gas outlets at the upper end of their rear wall, passagewaymeans extending from said outlet, vertically extending steam generatortubes lining the front wall of each furnace and the front portion ofeach side wall thereof, a steam generating circuit into which thesetubes are connected and which includes a steam and water drum positionedintermediate said furnaces adjacent the upper end thereof, each furnacehaving panel means formed of vertically extending steam generator tubespositioned adjacent said front wall at the lower region thereof withsaid panel means extending into the furnace from said front wall anddisposed in spaced relation across said front wall thereby forminghorizontally spaced chambers in the furnace, means for horizontallyintroducing fuel and air into said horizontally spaced chambers,convection heat exchange surface disposed in said passageway means andincluding superheating surface for superheating steam generated in saidsteam generating tubes, said superheater furnace :having the remainingportion of its Walls lined with superheater tubes extending verticallythroughout the length thereof, means for conveying steam from saidconvection superheater surface to a given number of these superheatertubes on the side walls so that the steam is conveyed down therethroughwith these tubes being iutercalated among the remaining superheatertubes on the side Walls, means for conveying this steam after passingdown through these superheater tubes on the side walls up through theremaining superheater tubes, the rear furnace wall being lined withvertically extending superheater tubes that extend from the upper to thelower end of the furnace, means for conveying the steam after traversingsaid remaining superheater tubes down through these tubes lining therear wall, header means at the lower end of the furnace communicatingwith these last mentioned superheater tubes, said reheater furnacehaving the remaining portion of its side walls lined with reheater tubesthat extend vertically from the lower end to the upper end thereof,header means receiving steam to be reheated and communicating with thelower end of these reheater tubes so that the steam is conveyed upwardlytherethrough, the rear furnace wall being lined with reheater tubes thatextend vertically from the upper to the lower end thereof, means forconveying this steam after passing up through the reheater tubes in theside walls to the upper end of the reheater tubes on the rear wall forpassage downwardly therethrough, and header means at the lower end ofthe furnace communicating with the last mentioned reheatertubes andreceiving the steam therefrom.

2. The organization of claim 1 wherein the superheater tubes on thefurnace side walls are alternately arranged with regard to steam fiow.

3. The organization of claim 1 wherein each of the furnaces is of thehopper bottom type with the tubes lining the side walls being slopedinwardly at the lower region of the walls to form said hopper bottom.

4. A vapor generator operating on the reheat cycle comprising a pair ofseparate upright furnaces of generally rectangular transverse sectionproviding a front, rear and two side walls with each of said furnaceshaving radiant vapor heating surface lining some of the walls thereofand with each furnace being fired in the lower region thereof, theradiant vapor heating surface of one furnace being comprised ofsuperheating surface and that of the other furnace being comprised ofreheating surface, said superheating surface including tubes extendingverti-cally along the inner surface and throughout the length of each ofthe side walls of said one furnace, means for delivering vapor to theupper ends of about one half of these tubes on each side Wall, a commonheader for the tubes of each side wall and to which the lower ends ofthe superheating tubes on the respective side Walls are connected, therear furnace wall of said one furnace having its inner surface linedwith vertically extending superheating tubes, means delivering the vaporfrom the upper ends of the other half of the superheater tubes on eachside wall and to the upper end of these rear wall tubes, means forconveying vapor from the lower end of these last-mentioned tubes to apoint of use, said reheating surface including tubes extendingvertically along the inner surface of each side wall and the rear well,these side wall tubes being in serial flow relation with the rear Walltubes and with the arrangement being such that the steam to be reheatedenters the lower ends of the side Wall tubes passes up therethrough anddown through the rear wall tube and then to a point of use.

5. A vapor generator operating on the reheat cycle comprising a pair ofseparate upright furnaces of generally rectangular transverse sectionproviding a front, rear and two side walls with each of said furnaceshaving radiant vapor heating surfaces lining some of the walls thereofand with each furnace being fired in the lower region thereof, vaporgenerating tubes disposed on at least some of the Walls of said furnacesand connected into a vapor generating circuit, said furnaces being inspaced relation for the interposition of a vapor and liquid drumtherebetween, a vapor and liquid drum mounted in the space between saidfurnaces at the upper region thereof with the axis of the drumhorizontal and parallel with the furnace walls delineating said spaceand with the drum forming part of the vapor generating circuit, theradiant vapor heating surface of one furnace being comprised ofsuperheating surface and that of the other furnace being comprised ofreheating surface, said superheating surface including tubes extendingvertically along the inner surface and throughout the length of each ofthe side walls of said one furnace, means for delivering vapor to theupper ends of about one half of these tubes on each side Wall, a commonheader for the tubes of each side wall and to which the lower ends ofthe superheating tubes on the respective side walls are connected, therear furnace wall of said one furnace having its inner surface linedwith vertically extending superheating tubes, means delivering the vaporfrom the upper ends of the other half of the superheater tubes on eachside wall to the upper end of these rear wall tubes, means for conveyingvapor from the lower end of these last-mentioned tubes to a point ofuse, said reheating surface including tubes ex- 7 8 tending verticallyalong the inner surface of each side wall References Qlted in the fileof this patent and the rear wall, these side wall tubes being in serialflow relation with the rear wall tubes and with the arrange- UNITED STA1 ES PATENTS merit being such that the steam to be reheated enters the2,609,798 nerly Sept. 9, 1952 lower ends of the side wall tubes passesup therethrough 5 2,737,160 AfmaCOSt 6t 1 M i 1956 and down through therear wall tubes and then to a point 2,731,745 Armfl s 6 31 1957 of use.2,863,424 Koch Dec. 9, 1958

1. A VAPOR GENERATOR OPERATING ON THE REHEAT CYCLE COMPRISING A PAIR OFUPRIGHT FURNACES GENERALLY POLYGONAL IN TRANSVERSE SECTION AND DISPOSEDIN SIDE BY SIDE SPACED RELATION, ONE OF SAID FURNACES COMPRISING AREHEAT FURNACE AND THE OTHER A SUPERHEAT FURNACE, SAID FURNACES HAVINGCOMBUSTION GAS OUTLETS AT THE UPPER END OF THEIR REAR WALL, PASSAGEWAYMEANS EXTENDING FROM SAID OUTLET, VERTICALLY EXTENDING STREAM GENERATORTUBES LINING THE FRONT WALL OF EACH FURNACE AND THE FRONT PORTION OFEACH SIDE WALL THEREOF, A STREAM GENERATING CIRCUIT INTO WHICH THESETUBES ARE CONNECTED AND WHICH INCLUDES A STREAM AND WATER DRUMPOSITIONED INTERMEDIATE SAID FURNACES ADJACENT THE UPPER END THEREOF,EACH FURNACE HAVING PANEL MEANS FORMED OF VERTICALLY EXTENDING STEAMGENERATOR TUBES POSITIONED ADJACENT SAID FRONT WALL AT THE LOWER REGIONTHEREOF WITH SAID PANEL MEANS EXTENDING INTO THE FURNACE FROM SAID FRONTWALL AND DISPOSED IN SPACED RELATION ACROSS SAID FRONT WALL THEREBYFORMING HORIZONTALLY SPACED CHAMBERS IN THE FURNACE, MEANS FORHORIZONTALLY INTRODUCING FUEL AND AIR INTO SAID HORIZONTALLY SPACEDCHAMBERS, CONVECTION HEAT EXCHANGE SURFACE DISPOSED IN SAID PASSAGEWAYMEANS AND INCLUDING SUPERHEATING SURFACE FOR SUPERHEATING STREAMGENERATED IN SAID STREAM GENERATING TUBES, SAID SUPERHEATER FURNACEHAVING THE REMAINING PORTION OF ITS WALLS LINED WITH SUPERHEATER TUBESEXTENDING VERTICALLY THROUGHOUT THE LENGTH THEREOF, MEANS FOR CONVEYINGSTREAM FROM SAID CONVECTION SUPERHEATER SURFACE TO A GIVEN NUMBER OFTHESE SUPERHEATER TUBES ON THE SIDE WALLS SO THAT THE STREAM IS CONVEYEDDOWN THERETHROUGH WITH THESE TUBES BEING INTERCALATED AMONG THEREMAINING SUPERHEATER TUBES ON THE SIDE WALLS, MEANS FOR CONVEYING THISSTREAM AFTER PASSING DOWN THROUGH THESE SUPERHEATER TUBES ON THE SIDEWALLS UP THROUGH THE REMAINING SUPERHEATER TUBES, THE REAR FURNACE WALLBEING LINED WITH VERTICALLY EXTENDING SUPERHEATER TUBES THAT EXTEND FROMTHE UPPER TO THE LOWER END OF THE FURNACE, MEANS FOR CONVEYING THESTREAM AFTER TRANVERSING SAID REMAINING SUPERHEATER TUBES DOWN THROUGHTHESE TUBES LINING THE REAR WALL, HEADER MEANS AT THE LOWER END OF THEFURNACE COMMUNICATING WITH THESE LAST MENTIONED SUPERHEATER TUBES, SAIDREHEATER FURNACE HAVING THE REMAINING PORTION OF ITS SIDE WALLS LINEDWITH REHEATER TUBES THAT EXTEND VERTICALLY FROM THE LOWER END TO THEUPPER END THEREOF, HEADER MEANS RECEIVING STREAM TO BE REHEATED ANDCOMMUNICATING WITH THE LOWER END OF THESE REHEATER TUBES SO THAT THESTEAM IS CONVEYED UPWARDLY THERETHROUGH, THE REAR FURNACE WALL BEINGLINED WITH REHEATER TUBES THAT EXTEND VERTICALLY FROM THE UPPER TO THELOWER END THEREOF, MEANS FOR CONVEYING THIS STREAM AFTER PASSING UPTHROUGH THE REHEATER TUBES IN THE SIDE WALLS TO THE UPPER END OF THEREHEATER TUBES ON THE REAR WALL FOR PASSAGE DOWNWARDLY THERETHROUGH, ANDHEADER MEANS AT THE LOWER END OF THE FURNACE COMMUNICATING WITH THE LASTMENTIONED REHEATERTUBES AND RECEIVING THE STREAM THEREFROM.